Conventionally, in an evaporated-gas purge system, the evaporated gas is adsorbed at a canister to prevent an evaporated gas generated in a tank from being leaked into the atmosphere. Further, since a purge control valve provided in a purge passage communicating with the canister and an intake system of an internal combustion engine is opened, the evaporated gas adsorbed at the canister is purged to the intake system according to a negative pressure of the intake system. The negative pressure corresponds to an intake negative-pressure. It is necessary to detect a leakage of the evaporated gas in an early stage to prevent from leaving a leakage state of the evaporated gas purged from the evaporated-gas purge system to the atmosphere.
For example, JP-H09-296753A (U.S. Pat. No. 5,758,628 A) discloses a technology that the purge control valve is opened to introduce the intake negative-pressure into the tank while the internal combustion engine is operating. Then, the purge control valve is closed, and a variation amount of a pressure in the tank is detected. Further, an abnormality diagnosis is executed to determine whether an abnormality such as the leakage occurred by comparing the variation amount to a determining value. According to JP-H09-296753A, when a concentration of the evaporated gas flowing from the canister into an intake pipe is high while the internal combustion engine is operating, an abnormality determining condition is not met, and the abnormality diagnosis is not executed. Therefore, the evaporated gas of high concentration is prevented from flowing from the canister into the intake pipe while the abnormality diagnosis is executed, and the internal combustion engine is prevented from failing.
However, when an evaporated-gas amount in an evaporation system is excessive such that an evaporated-gas adsorbed amount in the canister becomes a saturation value, the evaporated gas unable to be completely adsorbed at the canister may flow into a leakage-checking module according to a negative-pressure pump. A state that the evaporated-gas adsorbed amount becomes the saturation value is referred to as a saturation state corresponding to a breakthrough state. In this case, when a leakage diagnosis is executed, a diagnosis accuracy of the leakage diagnosis may be lowered due to an affect of a pressure variation according to the evaporated gas. Therefore, it may be erroneously determined about the leakage.
Further, according to JP-H09-296753A, when the concentration of the evaporated gas flowing from the canister into the intake pipe is high while the internal combustion engine is operating, the abnormality diagnosis is not executed in a case where the internal combustion engine is stopped. However, in a case where the concentration of he evaporated gas is low, the evaporated gas may be readily generated after the internal combustion engine is stopped. Therefore, the evaporated-gas amount is increased, and the evaporated-gas adsorbed amount of the canister may be in the saturation state. In other words, since the concentration of the evaporated gas is high when the internal combustion engine is operating, it may be erroneously determined about the leakage, even though the leakage diagnosis is not executed when the internal combustion engine is stopped.